This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates generally to the art of denitrification and more particularly to the art of denitrifying waste solutions having high nitrate concentration.
Waste solutions containing high concentrations of nitrate and nitrite ions represent a serious threat to the ecological balances which exist in nature. Accelerated eutrophication of lakes and streams is often caused by discharging conventionally treated waste effluent into the surface waters because these wastes contain quantities of nitrogen and phosphorus which can promote excessive algae production. Aside from being a major nutrient for algae production, nitrogen in the form of ammonia is toxic to aquatic life and can react with chlorine to form chloramines which are toxic to certain fishes. Water for livestock is considered unsafe at nitrate nitrogen concentrations exceeding 10 ppm. These concentratins can cause methemoglopinemia, vitamin A diffficiency, loss of milk production, thyroid disturbances and reproductive difficulties. Nitrite wastes are considered to be injurious to several species of fish at concentrations on the order of 5 ppm nitrite nitrogen. Complete denitrification (conversion of nitrate or nitrite to elemental nitrogen gas) prior to releasing wastes to surface waters is thus desirable.
Considerable research has been conducted regarding the biological denitrification of low level nitrate and nitrite wastes such as that of municipal wastes and agricultural wastewaters. Such wastes seldom exceed 60 ppm nitrate nitrogen. In the past, units containing activated sludge having bacteria therein have been used for denitrifying such wastes. Such activated sludge units utilize dentrifying bacteria which are suspended by mixing with the wastewater. The mixture of activated sludge in the wastewater is then passed to a settling tank for separation of the final effluent in the activated sludge. The sludge which settles from the settling tank is normally recycled with the further wastes. Packed bed reactors have also been utilized for conventional denitrification using denitrifying bacteria. A packed bed denitrification unit is an enclosed reactor containing submerged inert packing material which provides surface area for bacterial growth. Packed bed systems are operated both aerobically and anaerobically under flooded conditions with either an upward or downward flow.
While the prior art units are satisfactory for handling the low nitrate concentration levels of minicipal and agricultural wastes, they have been found unsatisfactory for handling high concentrations of nitrate wastes. In fact very little effort has been directed towards removing nitrates from wasterwaters containing concentrations of nitrate nitrogen in excess of 1000 ppm. Wastewater streams containing this magnitude of nitrates are generated in fertilizer and explosive manufacturing operations. Large quantities of wastewater effluent containing high nitrate concentrations are generated in nuclear fuel processing operations and at uranium oxide fuel fabrication plants.